1. Field of the Invention
The present invention relates to charging circuits of secondary batteries for use in small mobile electronic devices such as mobile phones and notebook computers, a method of controlling the operation of the charging circuits, and power supply units and, in particular, to a charging circuit of a secondary battery which charges the secondary battery using an energy-limited fuel cell as a power supply, a method of controlling the operation of the charging circuit, and a power supply unit.
2. Description of the Related Art
Recently, there has been a growing interest in the global environment and energy, and in such a circumstance, fuel cells having a low environmental impact have been adopted as a power supply for mobile devices. With mobile phones in particular, it can easily be predicted that, as a result of the introduction of digital terrestrial broadcasting and next-generation communication standards, the standby time of mobile phones will be shortened due to a more-than-ever increase in the electric power consumption of the devices. With respect to notebook computers, the extension of their continuous operating time is hoped for and the adoption of batteries having a high energy density will be necessary along with the low power consumption of the devices.
Although fuel cells have a high energy density, their generated voltages per cell are generally low, ranging from 0.3 V to 1.23 V. Generally, it is not possible to drive a load with such low voltages. Furthermore, since fuel cells have an extremely low power density, they are available in the so-called hybrid composition which boosts a generated voltage with a voltage boosting circuit, stores the generated energy in secondary batteries having a high power density connected parallel to a load, and supplies power from the secondary batteries to the load. The transfer of electricity from the fuel cells to the secondary batteries is performed through a charging circuit.
FIG. 10 is a flowchart exemplifying a method of controlling charging of a typical charging circuit which charges a lithium-ion battery using an AC adapter. Referring now to FIG. 10, a description will be made about a method of controlling the charging of the typical charging circuit.
In FIG. 10, when a charging circuit is connected to an AC adapter having a lithium-ion battery mounted thereon, the process proceeds from standby to preliminary charging. Furthermore, when the voltage of the lithium-ion battery rises, the process then proceeds from the preliminary charging to boost charging. Generally, the boost charging performs the so-called constant current and constant voltage charging which is controlled to charge with a constant current value and with a constant voltage, respectively, so as not to exceed the rated voltage of the lithium-ion battery.
When the voltage of the lithium-ion battery gets close to a previously set constant voltage, the charging current is reduced. When the charging current has become less than a predetermined charging completion current value i3 to detect a full charge, it is determined that the charging has been completed, and the charging current is controlled to become 0 mA. In the charging completed state, the lithium-ion battery is discharged by the operation of the device, and when the voltage of the lithium-ion battery is reduced, the boost charging is started again. At this time, the charging control circuit of the charging circuit monitors the voltage of the lithium-ion battery in preparation for the recharging, and the function constituting the charging circuit also operates.
Note that, in a separate invention, there is provided a mobile electronic device having a power supply unit which uses a fuel cell capable of lengthening the available operating time of the device until refueling and also preventing the device and the user from being damaged/injured in case of suddenly running out of fuel. Reference is made to JP-A-2004-227805, for example.
With the typical method as shown in FIG. 10, the charging control circuit of the charging circuit still operates even in the charging completed state where no charging is performed, resulting in the consumption of scarce current. However, since an AC adapter can supply nearly unlimited electric power, it is not necessary to stop the charging control circuit. When a fuel cell is used instead of an AC adapter, on the other hand, it has a limited energy supply. Therefore, in a case where the charging control circuit operates even at the time when no charging is performed, scarce current is supplied to the charging control circuit, thereby shortening the available operating time of the fuel cell. Furthermore, when a fuel cell is used for a mobile device, the standby time of the mobile device becomes shortened.